The present invention according to first and second aspects generally relates to a method and a system, respectively, for counteracting interference between cells in a cellular mobile radio system including a plurality of radio base stations and mobile radio stations. More particularly, the radio base stations are of the kind using, as units of transmission; bursts organized in groups, each group forming a frame of the type used by a Time Division Multiple Access TDMA system, and each burst containing a known sequence of data bits. At least two of the radio base stations are supposed to be co-channel radio base stations employing a same frequency. According to a third aspect the invention also relates to a radio base station for use in a cellular mobile radio system of the kind mentioned.
There are a number of publications teaching synchronization of the point of time for sending information (e.g. time slots, frames, training sequences) so as to increase the correlation between the cells (or the radio transmitters). This is true e.g. in "simulcast" systems and in systems offering macro diversity (e.g. CDMA systems). U.S. Pat. No. 5,206,855 and U.S. Pat. No. 4,972,507, to be dealt with in some more detail below, describe systems wherein transmission of time slots is planned so as to avoid that cells/stations located close together send simultaneously and on the same frequency.
U.S. Pat. No. 5,206,855 discloses transmission of messages in a system using several frequencies. The messages are transmitted sequentially in such a way that messages from one cell are not transmitted simultaneously with transmission from another cell.
U.S. Pat. No. 4,972,507 relates to a method for transmitting data between the base station and the user unit in a mobile telephony system. All user units in a cell are interrogated by the base station transmitting an interrogation signal. The user units respond by sending a short message where each unit has a unique delay for avoiding that messages interfere.
U.S. Pat. No. 5,473,612 discloses how to decrease the risk for erroneous detection of a data packet in a radio communication system. The described invention is based upon the fact that packets from different stations have different delay. At reception from a certain station the synchronization sequence is searched in a time window corresponding to the delay of said station.
WO 95/35,601 discloses a method for minimizing interference from surrounding cells by using four different frequencies and directional antennas.
WO 95/17,048 discloses a radio telephony system where "co-channel interference" is reduced by assigning channels dependent on the position of the mobile unit and controlling the output power so as to make mobiles located within border areas to send with a lower power.
WO 94/30,024 describes a method for synchronizing two base stations in a CDMA system. The purpose of the synchronizing is to obtain so-called "macro diversity".
Further publications showing a more general stand of the art are U.S. Past. No. 4,642,806, EP 208,021, DE 29,43,115, WO 95/08,901, U.S. Pat. No. 5,124,698 and U.S. Pat. No. 5,509,016.
Below a number of aspects within the so-called GSM-system will first be dealt with. To the extent that some of these aspects are only mentioned below, without being described in detail, reference can be made to "The GSM System for Mobile Communications, A comprehensive overview of the European Digital Cellular Systems", by Michel MOULY and Marie-Bernadette PAUTET, also being publishers, International Standard Book No. 2-9507190-0-7.
In high traffic areas, such as large cities, the capacity of a cellular system is limited by its own interferences caused by frequency reuse. The relative interference ratio, expressed as C/I, where C is the Carrier level and I is the Interference level, may vary a lot between calls. C changes with the mobile station position relative to the base station, with the amount of obstacles between them, etc.; I changes depending on whether the frequency is being used by another call in some nearby cell, and it also varies according to the distance to the interfering source, its level, etc.
A basic concept of the GSM transmission on the radio path is that the unit of transmission, called burst, is a series of e.g. about 150 modulated bits. The GSM bursts are organized in groups of 8, such a group being referred to as a TDMA frame (TDMA--Time Division Multiple Access). Bursts have a finite duration, and occupy a finite part of the radio spectrum. They are sent in time and frequency windows, or slots. Precisely, the central frequencies of the slots are positioned every 200 kHz within the system frequency band (FDMA aspect), and they recur in time every 0.577 ms, or more exactly, every 15/26 ms (TDMA aspect). All slot time limits regarding different carrier frequencies are simultaneous in a given cell.
Within the time interval of a time slot, the amplitude of transmission rises from a starting value of 0 to its nominal value. The signal phase is modulated to transmit a packet of bits. After that, the amplitude decreases until it reaches 0.
The packet of bits used to modulate the signal phase of a burst includes a training sequence besides the variable part of the information, plus "0" bits at each end. The training sequence is a sequence of 26 bits known by the receiver. The signal resulting from the transmission of this training sequence allows the receiver to determine very precisely the position of the useful signal inside a reception time slot, and to have an idea of the distortion caused by transmission. These informations are of prime importance to obtain good demodulation performance.
There are several such training sequences defined in GSM. Thus, eight different training sequences have been specified.
One of the purposes of using training sequences is to obtain equalization.
If two signals, one desired and one interfering, arrive at a receiver at almost the same time, and their training sequences are the same, there is, in conventional receivers, no way to distinguish the contribution of each of them from the received signal. The situation is much clearer when the two training sequences differ, and are as little correlated as possible. Distinct training sequences are therefore allocated to channels using the same frequencies in cells which are close enough to interfere with one another.
The eight training sequences have been chosen for the special shape of their autocorrelation function, which is meant to ease some demodulation techniques.